Imagine that you are observing from a point on the equator. All stars are visible at some time of the year, having any declination from +90º to -90º. Now, imagine you move your observatory 20º to the North. The South celestial pole will also dip by 20º below your horizon, and with it, all the stars having a declination more negative than -70º... If you continue moving your observatory to the North, you will reach the North pole, from which any star with a negative declination is permanently below the horizon, and can't be observed.

Both declination and right ascension are fixed celestial coordinates, and they don't change in days... In the long term, they do, because of proper motions and the precession of the equinoxes, but -for normal use- they can be considered as fixed.